Philips Atlas of the Universe

Rings of Saturn

ATLAS OF THE UNIVERSE

T

he ring system of Saturn is unique, and quite unlike the

dark, obscure rings of Jupiter, Uranus and Neptune.

Saturn’s rings were first seen in the 17th century, and

Christiaan Huygens, in 1656, explained them; previously

Saturn had even been regarded as a triple planet.

There are two bright rings (A and B) and a fainter

inner ring (C) which was discovered in 1850, and is usually

known as the Crêpe or Dusky Ring because it is semi-

transparent. The bright rings are separated by a gap known

as Cassini’s Division in honour of G. D. Cassini, who

discovered it in 1675. Various fainter rings, both inside

and outside the main system, had been reported before the

Space Age, but there had been no definite confirmation.

The main system is relatively close to the planet, and lies

well within the Roche limit – that is to say, the minimum

distance at which a fragile body can survive without being

gravitationally disrupted; the outer edge of Ring A lies

at 135,200 kilometres (84,000 miles) from Saturn’s

centre, while Mimas, the innermost of the satellites known

before the space missions, is much further out at 185,600

kilometres (116,000 miles).

The full diameter of the ring system is about 270,000

kilometres (169,000 miles), but the thickness is no more

than a few tens of metres (60 to 100 feet). Represent

the full spread of the rings by the diameter of a cricket

or baseball field, and the thickness will be no more than

that of a piece of cigarette paper. This means that when

the rings are edgewise-on to us they almost disappear.

Edgewise presentations occur at intervals of 13 years

9 months and 15 years 9 months alternately, as in 1966,

1980 and 1995. This inequality is due to Saturn’s orbital

eccentricity.

During the shorter interval, the south pole is tilted

sunwards – in other words, it is summer in the southern

hemisphere – and part of the northern hemisphere is

covered up by the rings; during this time Saturn passes

through perihelion, and is moving at its fastest. During the

longer interval the north pole is turned sunwards, so that

parts of the southern hemisphere are covered up; Saturn

passes through aphelion, and is moving at its slowest.

The rings are at their most obscure when the Earth is

passing through the main plane or when the Sun is doing

so. It is wrong to claim that they vanish completely; they

can be followed at all times with powerful telescopes,

but they cannot be seen with smaller instruments, and at

best they look like very thin, faint lines of light.

No solid or liquid ring could exist so close to Saturn

(if, indeed, such a ring could ever be formed in the first

place). It has long been known that the rings are made up

of small particles, all moving round the planet in the

manner of tiny moons. There is no mystery about their

composition; they are made up of ordinary water ice.

 Aspects of the rings as

seen from the Hubble Space

Telescope annually from

1994 to the year 2000.

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